Composite structures are widely used in high performance applications because of their light weight, high strength, high stiffness and superior fatigue resistance. These structures broadly comprise a combination of dissimilar constituent materials bonded together by a binder, but are most commonly formed by a thermosetting resin matrix in combination with a fibrous reinforcement, typically in the form of a sheet or mat. Multiple plies of the matting are impregnated with a binder such as epoxy plastic resin or polyester resin, and formed into a “lay-up”. Pressure and heat are applied to the multi-layer part lay-up in order to compress and cure the plies, thereby forming a rigid structure.
Pressure is applied to the lay-up using a technique referred to as “vacuum bagging” wherein the lay-up part is placed inside a bag within which a vacuum is drawn. The vacuum within the bag results in the application of compaction pressure to the lay-up which assists in consolidating the plies. The bag vacuum also extracts moisture, solvents and volatiles from the curing composite, and urges the resin to both flow and be absorbed in the lay-up without hydraulic lock. Heat is simultaneously applied to the lay-up by placing the bagged part in an oven or autoclave.
In order to prepare a lay-up for vacuum bagging the part lay-up is placed on a tool base, and a number of layers of material are applied over the lay-up in a sequence referred to as a bagging schedule, following which a bag is placed over a part and sealed to the tool base. The vacuum bags are subject to tearing and the material used to seal the bags to the tool base is subject to leakage or even failure, in which event the bag vacuum is lost. In order to protect against bag or seal failure, part lay-ups are sometimes “double bagged”. Double bagging involves placing a first, inner bag over the lay-up which is then sealed to the tool base. A second, outer bag is placed over the inner bag and is also sealed to the tool base. This double bagging technique is time consuming because two bags must be separately formed, trimmed to dimension and sealed. An inordinate amount of sealing area is required which may pose difficulties where the surface area on the tool provided for the seal is relatively small. In order to overcome this problem, in some cases the surface area of the tool can be made larger, but this solution results in tooling that uses more material, has increased weight and greater tool mass. The greater tool mass increases the time required to heat the tool to temperature, thereby increasing the overall cure cycle time.
From the forgoing, it can be appreciated that the prior method of double bagging lay-up parts is both time consuming and requires additional materials. Accordingly, there is a need for a double bag and a vacuum bagging method that overcomes these problems. The present invention is directed towards satisfying this need.